Magnetically inductive flow meters such as known from U.S. Pat. No. 4,614,121 work on the principle that a magnetic field is produced of a given size and the voltage induced in the flowing liquid is measured transversely to the magnetic field.
For this purpose, the induction coil is alternately fed with positive and negative current pulses of a predetermined value. The pulses may be separated from each other by a passage through zero and/or by a pause between them. Measurement always takes place after the end of a pulse, after all transitory conditions have decayed. For example, the polarity of the current changes from eight to ten times per second. The induction coil has an inductance of, for example, 100 to 600 mHy. The energizing current is in the order of .+-.0.1 to 0.2 A.
If, during the operation of such a flow meter, the supply line is interrupted, for example because a plug is removed, a high voltage is set up at the point of interruption by reason of the inductance of the coil and this can lead to sparking. This is particularly so if the leads are first short circuited and then separated again, as is the case when the leads are accidentally cut by pliers. In this case, because of the small space between the leads, even lower voltages will produce a spark. It was therefore not possible to place a magnetically inductive flow meter in an explosion risk zone.